A hybrid electric vehicle (HEV) can selectively utilize different onboard energy sources in order to fully optimize fuel economy. Power sources typically include an internal combustion engine and at least one high-voltage motor/generator unit (MGU), which may draw power from and deliver power to a battery as needed. A full hybrid powertrain can be used to electrically propel the HEV in an electric-only (EV) mode, while a mild hybrid powertrain has less EV propulsion capability but retains certain fuel saving features, such as engine auto-stop/auto-start functionality and regenerative braking.
Engine auto-stop/auto-start functionality allows an HEV to shut off its engine when at a stop, and to instantly restart the engine when a brake pedal is released and an accelerator pedal is depressed with a sufficient amount of force. Power needed for cranking the engine can be provided by an auxiliary starter motor, or in the case of a belt alternator starter (BAS) system in some mild hybrid designs, via motor torque from the MGU. Once started, the engine of a BAS-equipped HEV delivers the required torque to a set of drive wheels for propelling the HEV.